1. Field of the Invention
The present invention relates to a hydraulic braking system for providing various braking force control modes in an automotive vehicle, and more particularly to a hydraulic braking system which has an auxiliary pressure source separate from a master cylinder or the like, and which is capable of controlling the braking force applied to each wheel individually.
2. Description of the Prior Art
Recently, an automotive vehicle is provided with a braking force control system for providing various control modes such as an anti-skid control mode, traction control mode, vehicle stability control mode and the like. The provision of those modes results in increase in cost, and decrease in space. Therefore, various systems have been proposed to provide a hydraulic braking system employed in the braking force control system at a relatively low cost, with a relatively small space occupied by the system, comparing with the prior systems. With respect to those systems, it has been proposed to employ inexpensive electromagnetic valves for a pressure control device and reduce the number of electromagnetic valves as small as possible.
In Japanese Patent Laid-open Publication No. 3-169769, for example, inexpensive valves are employed in an anti-skid control system, wherein hydraulic circuits communicating a master cylinder with wheel cylinders operatively mounted on road wheels of a vehicle are divided into two circuits so as to provide a diagonal dual circuits system. In each hydraulic braking pressure circuit, an inlet valve is disposed, while an outlet valve is disposed in each return circuit or passage. These inlet and outlet valves are controlled to be opened or closed, respectively, in response to each wheel speed. According to that system, in a pressure circuit which connects a pressure circuit which is provided between the outlet port of each inlet valve and a wheel cylinder for each front wheel, with a pressure circuit which is provided between the inlet port of each outlet valve and a wheel cylinder for each rear wheel, an open/close valve which is placed in the open position in its inoperative condition is disposed in each pressure circuit, and also a control circuit is provided for controlling the operation of each open/close valve individually so as to be capable of controlling the braking pressure individually in each of the wheel cylinders of the front and rear wheels which are paired with each other through the pressure circuit.
In FIG. 2 of the above-described publication, an apparatus having an auxiliary pressure source for returning the brake fluid has been disclosed, as shown in FIG. 25. A pair of auxiliary reservoirs 25, 26 are incorporated into the auxiliary pressure source 10a, and connected with return circuits 18a, 18b which communicate the outlet ports of outlet valves 21p, 22p ("p" is added after the numerals used in the publication to avoid confusing with the numerals used in the embodiments of the present invention), respectively. This arrangement has been explained in the publication as an example applied to a system for releasing the hydraulic pressure in the wheel cylinder under the condition of non-atmospheric pressure.
In Japanese Patent Laid-open (for PCT) Publication No. 5-507667, there is disclosed an apparatus for controlling the braking pressure in each wheel cylinder. An anti-lock oil pressure braking system disclosed in this publication has a separating valve for blocking a brake passage, a return passage for connecting a wheel brake with the inlet side of a pump, an outlet valve disposed in the return passage, and a pressure passage for connecting the pressurized side of the pump to a position between the separating valve and the wheel brake. In that publication, U.S. Pat. No. 4,636,009 has been employed as a prior art, and it is described that in view of the prior art having three-port three-position valves with complex structure, a method for providing a single electromagnetic control outlet valve for closing or opening the return passage, disposing a control valve in the brake passage, and supplying from the pump the pressurized fluid into the wheel brakes through a diaphragm, has been proposed. Furthermore, in that publication, a limiting element (9) has been disposed in the pressure passage (8), to provide means for introducing additional pressurized fluid into a control circuit downstream of the separating valve.
However, according to the apparatus disclosed in the Publication No. 3-169769, the braking force of each wheel may be controlled independently in such a limited sense that the braking pressure in the front wheel cylinder can not be held constant, when the braking pressure in the rear wheel cylinder is increased, and that the braking pressure in the rear wheel cylinder can not be held constant, when the braking pressure in the front wheel cylinder is decreased. That is, the selection timing of the so-called hold mode is limited. For example, when the rear wheel RR or RL does not tend to be locked, nor has been locked, the braking pressure in the rear wheel RR or RL must be increased according to the apparatus disclosed in the publication, so that the open/close valves 23p, 24p are opened. As a result, the braking pressure control for the rear wheels is dependent upon that for the front wheels, so that the property sufficient for the independent braking force control can not be obtained. The braking pressure at the one wheel's side (rear wheel's side) is affected by the pressure at another wheel's side (front wheel's side) in the same hydraulic circuit, so that the apparatus can be used only for the diagonal dual circuits system, but it can not be used for a front and rear dual circuits system.
Furthermore, according to the apparatus disclosed in the Publication No. 3-169769, the brake fluid discharged from the auxiliary pressure source 10a is supplied to a passage filled with a fluid pressurized at high pressure, so that the load caused against the auxiliary pressure source 10a is high. Therefore, the pressure source 10a may cause a large noise, and its durability will be shortened. While the braking pressure is being controlled by the inlet valve 20p, the outlet valve 22p and the open/close valve 24p, the variation of the braking pressure is directly transmitted to the pressure generator 1p. This results in a so-called kick-back phenomenon thereby to discomfort a vehicle driver depressing a brake pedal.
In order to prevent the kick-back phenomenon and reduce the noise caused by the above-described valve operation, it is effective to provide a closed circuit including the wheel cylinders, in the hydraulic braking pressure circuit, and control the braking pressure within the closed circuit. In the apparatus as shown in FIG. 2 of the Publication No. 3-169769, therefore, it is necessary to arrange the apparatus as shown in FIG. 26, with the electromagnetic valves 27p, 28p added, and make the inlet valve 20p function as a cut-off valve for blocking the communication with the pressure generator 1p. The number of electromagnetic valves is relatively small in comparing with the prior apparatus. Even if the apparatus is constituted as described above, however, the selection of the hold mode is limited.
Furthermore, in the case where the braking pressure has to be controlled without depressing the brake pedal, such as in the traction control mode or vehicle stability control mode, a hydraulic pressure source is needed in addition to the master cylinder or the like. When a conventional auxiliary pressure source is disposed in the circuit, however, it will become necessary to provide further a pre-charging pump or the like for ensuring a necessary increasing rate of pressure at the time of starting the pressure control. This will also result in increase in cost. It is, therefore, effective to maintain the auxiliary pressure source driven at relatively low load.
According to the apparatus disclosed in the Publication No. 5-507667, the separating valve (4) and the outlet valve (6) must be provided for each wheel to be controlled, so that the number of electromagnetic valves can not be reduced. One of the inventors in the present application has proposed in the separate U.S. patent application, an apparatus as disclosed in FIG. 27 to decrease and re-increase the braking pressure in each wheel cylinder in one pressure circuit system, and decrease its cost. That is, in the one pressure circuit system, only a single three-port two-position electromagnetic valve (electromagnetic changeover valve) and a pair of two-port two-position electromagnetic valve (normally open electromagnetic shut off valve) are included, and the brake fluid is supplied by a fluid pump to a position between the latter valve and each wheel cylinder through an orifice and a check valve, respectively. However, in this apparatus, the three-port two-position electromagnetic valves 14a, 34a are necessitated, and the hold mode in each wheel cylinder is provided for limiting the braking pressure within a certain pressure variation with the two-port two-position electromagnetic valve opened or closed, without shutting off the braking pressure. Therefore, in the case where it is necessary to block the pressure passage completely in the hold mode, the apparatus as shown in FIG. 27 is not appropriate. It is preferable that the two-port two-position electromagnetic valve is employed, and the number of valves is as small as possible, because the more the number of ports and positions of the valve is, the higher the cost of the electromagnetic valve is. Sometimes, a pressure sensor which is expensive is disposed in the vehicle stability control system, but the provision of the pressure sensor results in increase in cost. It is, therefore, preferable that the number of such expensive sensors is as small as possible. Thus, although the wheel speed sensor for detecting the wheel speed is necessary, it is preferable to provide a desired function without employing the pressure sensor or the like.